(1) Field of the Invention
This invention relates to pumping oil wells and more particularly to abrasion and corrosion of sucker rod couplings. Applicant submits that manufactures and users of oil well sucker rods and sucker rod couplings would be considered as having ordinary skill in this art.
(2) Description of the Related Art
Oil is removed from the ground using a pump-jack. This equipment is mounted on the surface of the earth above an oil reservoir. The pump-jack is connected to a down-hole pump using a sucker rod string. The sucker rod string includes several sucker rods, each sucker rod connected to the next by a coupling. Thus, oil is pumped from the reservoir to the surface for collection.
During the pumping action, the sucker rod string reciprocates in a tubing string. Due to deflection in the tubing string, the sucker rod string wears against the internal diameter of the tubing string. Even though lubricant in the form of crude oil is present in the annulus created between the sucker and tubing strings, abrasion occurs to the rods, couplings, and tubing.
Included in the crude oil are dissolved salts and undissolved minerals. When these come between the rods, couplings, and tubing, abrasion occurs. Through time, abrasion will decrease the rod and coupling diameters to a point that they will break under the stress upon the sucker rod string. In addition to the damage done to the sucker rod string, the abrasion will also wear away the tubing wall.
When a hole develops in the tubing wall, pressure inside the tubing string is lost, and the crude oil is pumped into the annulus created between the tubing and casing.
When a rod parts, a coupling breaks, or holes in the tubing occur, the sucker rods and/or tubing must be pulled from the well and inspected. Defective rods, couplings, and tubing must be replaced. The resultant pulling unit costs and down-time are a great expense for the pumping unit operator.
At the same time that abrasion is destroying the sucker rods, rod couplings, and tubing; all are being attacked by harsh, down-hole chemicals. Dissolved gasses and salts in the crude oil, in combination with the water, will destroy unprotected rods, couplings, and tubing, through corrosion. When sufficient corrosive attack has occurred, the sucker rods and couplings will break under the weight of the sucker rod string. The rods, rod couplings, and tubing must be replaced, due to corrosion.
Mild steel sucker rod couplings are the most commonly used couplings today. These couplings are beveled on both ends to preclude gouging of the tubing wall. The sucker rod pin ends are screwed into the coupling, using a specified torque, such that they shoulder up to the coupling face. The coupling is usually threaded throughout its entire length. The diameter of the coupling will vary, dependent upon the diameter of the sucker rod for which the coupling was manufactured. Precise machining of the radial face of the coupling is required to insure uniform contact with the face of the sucker rod pin end, around its circumference. Uneven pressures at this junction will lead to pin end failures in service.
Because these couplings are made of mild steel, they are subject to corrosive attack by down-hole chemicals. They are also subject to abrasion, when rubbing against the tubing as they reciprocate in the tubing string. Abrasion is magnified because the hardness of these couplings is the same as the mild steel tubing in which they are used. (Metals of similar hardness will abrade one another.)
To overcome the abrasion and corrosive attack of the mild steel sucker rod couplings, the oil industry capitalized on the relationship that "metals of dissimilar hardness will polish one another". Sucker rod couplings with a mild steel core and a stainless steel jacket were developed to take advantage of this relationship. They were first used in pumping wells which were highly deviated and therefore, caused excessive abrasion of the mild steel sucker rods, couplings, and tubing. Special grades of stainless steel were chosen, since stainless steel has greater resistance to corrosive attack, and since it has a greater hardness than mild steel. These rod couplings were prepared by machining away some of the original diameter of the mild steel couplings and depositing a jacket of stainless steel, using an arc plasma spray technique. The final diameter was the same as the original diameter. These stainless steel jacketed couplings had to be chamfered on the ends, since they would easily cut away the mild steel tubing as they reciprocated in the tubing string. The internal thread configuration was not altered.
This abrasion resistance technique proved to be beneficial in highly deviated pumping wells. A significant reduction in abrasion was noted. The stainless steel jacket was also impervious to attack by the down-hole chemical environment. In cases where the couplings were not chamfered on the ends, the tubing was quickly cut away, due to the greater hardness of the stainless steel jacket. In addition to this problem, the manufacturing process for these couplings was considerably more expensive than for mild steel couplings. These couplings were also difficult to install and remove. Rod tongs and wrenches could not grip the stainless steel jacketed couplings, because of their surface hardness. Both would slip as torque was applied to install or remove these couplings.
Another approach to solving the problems of abrasion and corrosion of sucker rod couplings is described in U.s. Pat. No. 4,602,807 to BOWERS. Using this technique, a sucker rod coupling is first machined to decrease its diameter. Then, one or more grooves are cut into the external diameter of the coupling. Special retaining ridges are also left on the ends of the couplings. Once machine, each coupling is given a jacket of polyurethane, using an injection molding technique. One set of expensive molds must be made for each coupling diameter. This polyurethane jacket is not bonded ot the sucker rod coupling. Without the special grooves and ridges, the polyurethane jacket would slip from its protective position around the coupling. The combination of expensive machining with the expensive molds for the injection molding process, make this an expensive sucker rod coupling. Since the polyurethane jacket is not bonded to the sucker rod coupling surface, corrosive chemicals will spread under the polyurethane jacket, attacking the metal substrate. This polyurethane jacket only extends to the radial face of the coupling. A metal-to-metal seal is formed when the coupling is installed on the sucker rod pin end. While this technique provides greater abrasion resistance than bare, mild steel sucker rod couplings, it will evantually fail, due to corrosive attack of the mild steel coupling core.
In yet another approach, a mild steel coupling is machined to decrease its diameter throughout the center section of the coupling. Ridges are left on both ends to hold the abrasion resistant, injection molded plastic protector in place. Once machined, the plastic is molded into the recessed area of the coupling to have a larger diameter than the original diameter and the retained ridges. The sections of greater diameter provide abrasion resistance where the sucker rod couplings make contact with the inside diameter of the tubing string. The mild steel ends are left unprotected. Corrosive chemicals will attack the mild steel ends on these couplings. Since this plastic is not bonded to the coupling, corrosive fluids can spread between the molded plastic jacket and the mild steel coupling body. When installed on a sucker rod, a metal-to-metal seal is formed between the face of the sucker rod pin end and the coupling shoulder. Besides being subjected to corrosion at this point, the corrosive down-hole fluids will spread inside the coupling, attacking it from the inside. These fluids will also attack the pin end of the sucker rod. Corrosion within the recessed area of the sucker rod pin ends is responsible for many sucker rod breaks in service. Less then precise machining of the sucker rod pin end face of the coupling shoulders will leave gaps between the rod and the coupling. Corrosive fluids can penetrate the coupling more easily when this condition exists. Imprecise machining can also lead to localized stresses on the sucker rod pin ends. These stresses will eventually cause the pin ends to break off the sucker rods, or the couplings to break into.
Couplings which have an injection molded, abrasion resistant jacket around them have met with limited success due to the problem already mentioned: Manufacturing expense, lack of a bond between the plastic jacket and the coupling body to preclude the spread of corrosive chemicals between them, and lack of an improved sealing surface of the shoulders of the couplings to prevent the migration of corrosive fluids into the hollow core of the couplings.
While stainless steel jacketed couplings have provided more abrasion resistance than mild steel couplings, and even though they are more resistant to down-hole chemicals, there are still shortcomings with this style of sucker rod couplings. Problem areas include: Manufacturing costs, ability to install and remove because of their surface hardness, and the lack of an improved sealing surface on the shoulders of the couplings to prevent the spread of corrosive chemicals into the hollow, mild steel core of the couplings.
The injection molded, plastic jacketed, and stainless steel jacketed couplings have another common deficiency: they can both unscrew down-hole becaause they are not locked in place on the sucker rod pin ends. Precise machining of the sucker rod pin end face and coupling shoulders is required in order to achieve a good seal around the coupling circumference. A metal-to-metal friction-fit seal is formed to hold the coupling and sucker rod together. Due to the weight of the sucker rod string in combination with the pounding of the rods and couplings against the tubing wall, the rods and couplings will often unscrew.
Fusion bond applied power plastic coatings have been used for many years, to prevent corrosion of sucker rods. These coatings are applied over abrasive blast-cleaned, mild steel sucker rods, as well as arc plasma sprayed stainless steel jacketed sucker rods. To have utility as a sucker rod coating, the coating resin must withstand the attack by corrosive down-hole chemicals. It must also bond to the sucker rod body and not blister or disbond in-service. The resin system must have a little flexibility, since the sucker rods will bend and flex in-service. Abrasion resistance is also required, to keep the coating from being worn away as the coated sucker rods reciprocate in the tubing string. Resistance to mechanical impact is a concern, since the coated sucker rods will slap against the tubing wall. Excessively high impact resistance, however, is not the primary requirement for a sucker rod coating. To fulfill these requirements, fusion bond coatings of the epoxy and modified-epoxy classes are used. These coating resin classes provide the required protection for sucker rod body. They are applied along the body of the rod, from one pin end face to the other. No coating is deposited on the sucker rod joint faces. Special masks are used to prevent coating deposition at this point, since it will adversely effect the ability of the sucker rod and coupling to form a friction-fit, metal-to-metal seal. Because these coatings resist compression and are hard and smooth, a coupling would unscrew from the sucker rod, if this type resin were present between the coupling shoulder and the rod face. Coatings of the epoxy and modified-epoxy classes are applied, mostly, in a fustion bond, powder form, (a) via electrostatic spray, or (b) hot flocking.
If coating resins with the physical and chemical properties of these sucker rod coatings were used to protect the sucker rod couplings, they would fail, due to their poor impact and chip resistance. For this reason, coated sucker rods are used in combination with stainless steel jacketed couplings, or injection molded, plastic jacketed couplings. Rod tongs and wrenches would crack, chip, and disbond coatings of the epoxy or modified-epoxy class, if they were present on the sucker rod couplings. Since the sucker rod couplings have a greater diameter than the sucker rods, most of the forces of down-hole impact are concentrated on the couplings. This concentrated down-hole pounding would destroy coatings of the epoxy or modified-epoxy class, if used to protect the exterior of the sucker rod couplings.
Before this application was filed, in addition to the background information the inventor was aware of, the inventor also caused searches to be made in the United States Patent and Trademark Office. This search developed the following references:
______________________________________ Inventor U.S. Pat. No. ______________________________________ Stephens 319,144 Bahr 1,134,092 Gray 1,703,232 Nathan 2,016,905 Crake 2,337,711 Ferguson 2,646,822 Cavallier 2,656,857 Holcombe 2,690,934 Johnson 3,462,175 Beene 3,675,950 Sable 3,942,824 Carlson 4,205,926 Kupcak 4,226,550 Garrett 4,436,118 Gallagher 4,484,833 Takahashi 4,577,664 ______________________________________
Applicant believes the information found in these patents is not as pertinent as the information specifically discussed above. However, applicant believes that the Examiner would be interested in anything found by a experienced patent searcher.